Server application for compressing images (lossless)?

In our web app, the administrator uploads lots of images (banners, product images, etc.). Then they're resized immediately to given format.

The problem is that they're not optimized in size. Especially banners, which don't get processed at all and get used as is.

I'm looking to set up a worker queue that will pick up queued task and optimized these image.

What server CLI application do you use for compressing such images?

It basically has to do the same as kraken.io, but locally on the server. The worker scripts will most likely be written in PHP, therefore some API wrapper would be nice. Otherwise, I guess there is the possibility of calling the system command and passing the arguments.